The terms microprocessor and microcomputer often are used interchangeably even though a microprocessor technically does not have any memory on the same chip with the processor. A microcomputer, on the other hand, includes a processor and at least some memory on the same chip. Since the present invention can be utilized with both microprocessors and microcomputers the terms will be used interchangeably herein. It is to be understood, however that use of one term includes the other unless the context indicates otherwise.
Microprocessors running application program, such as DOS based programs, have the ability to directly address only a limited amount of conventional memory. In the case of DOS based application processors that limit is 1 megabyte (MB) of memory. This limit of memory can be expanded by using an Expanded Memory System (EMS). Such an EMS is disclosed in the "AST-Enhanced Expanded Memory Specification (EEMS) Technical Reference Manual", 000408-001 A. September, 1986. A similar EMS is disclosed in "The MS-DOS Encyclopedia", Microsoft Press, Redmond, Wash., 1988. Duncan, Ray General Editor.
In general an EMS is provided by mapping 16 KB pages of expanded memory into a 64 KB area called the page frame, above the 640 KB memory boundary within the conventional memory. The DOS kernel does not take part in expanded memory manipulations and does not use expanded memory for its own purposes. The manipulation and management of the expanded memory is performed by an EMS controller. Typically, however, EMS memory is usually conceived as a fixed mass of RAM permanently attached to the DOS processor and controlled by a discrete realization of the EMS controller.
Extended memory (EXT) is that memory storage at addresses above 1 MB that can be accessed directly by a microprocessor running in protected mode. Protected mode operating systems, such as XENIX and OS/2, can use extended memory for execution of programs. DOS based application programs, on the other hand, run in real mode, and ordinarily cannot execute from extended memory or even address that memory for storage of data. Some microprocessors can theoretically have as much as 15 MB of extended memory installed in addition to the usual 1 MB of conventional memory address space.
Microprocessors can support three different types of memory. Conventional memory is the term used for the 1 MB of linear address space that can be accessed by the microprocessor running in real mode. Expanded memory (EXP) can be made available by mapping 16 KB pages into a 64 KB space within the 1 MB of conventional memory. Extended memory (EXT) relates to that memory above 1 MB that can be directly accessed by a microprocessor running in protected mode. The memory media can, of course, include a wide variety of structures, such as ROM, RAM, floppy disks, hard disks and others.